1. Field of the Invention
The present invention relates to the field of resistor-based memory circuits. More particularly, it relates to a method for precisely regulating the voltage applied across a resistive memory element during sensing.
2. Description of the Related Art
FIG. 1 shows one example of a resistor based memory. The memory includes a memory cell array 8 having a plurality of row lines 10 arranged in normal orientation to a plurality of column lines 12. Each row line is connected to each of the column lines by a respective resistor 14.
A Magnetic Random Access Memory (MRAM) is one approach to implementing a resistor based memory. In an MRAM, each resistive memory cell includes a magnetizable film. The resistance of the cell varies, depending on the magnetization state of the film. Logical data can be stored by magnetizing the film of particular cells so as to represent the logic states of the data. One resistance value, e.g., the higher value, may be used to signify a logic xe2x80x9cHIGHxe2x80x9d while the other resistance value, e.g., the lower value, may be used to signify a logic xe2x80x9cLOWxe2x80x9d. The stored data can be read by measuring the resistance of the cells, and interpreting the resistance values thus measured as logic states of the data.
For MRAM sensing purposes, the absolute magnitude of resistance need not be known; only whether the resistance is above or below a value that is intermediate to the logic high and logic low values. Nonetheless sensing the logic state of an MRAM memory element is difficult because the technology of the MRAM device imposes multiple constraints. In a typical MRAM device an element in a high resistance state has a resistance of about 1 Mxcexa9. An element in a low resistance state has a resistance of about 950 Kxcexa9. The differential resistance between a logic one and a logic zero is thus about 50 Kxcexa9, or 5% of scale. Rapidly distinguishing a 5% resistance differential on a scale of 1 Mxcexa9 with a minimum of circuitry is problematic.
Resistance is measured using Ohm""s Law which holds that resistance is equal to the voltage across a resistor divided by the current through the resistor. Generally one parameter is held constant while the other is measured. In MRAM applications voltage is typically held constant while current is measured. As a result, the quality of an MRAM resistance measurement depends in large part upon the ability to regulate the voltage applied across a resistive memory element during sensing.
Where a stabilized voltage is required, it is known to use a voltage follower circuit constructed by feeding back an output signal of a differential amplifier to an inverting input of the amplifier, and connecting a reference voltage to the non-inverting input of the amplifier. FIG. 2 shows one embodiment of a conventional voltage follower in block diagram form. A voltage follower exhibits a voltage gain of about 1, high input impedance, and low output impedance. A voltage follower circuit might thus be used to apply a voltage across a resistive memory element to sense the resistance of the resistor.
In practice, a conventional differential amplifier, such as might be found in a memory device, has offsets that affect the voltage across the memory cell. If an amplifier with an offset is used to provide a voltage across a MRAM resistive memory element for sensing purposes, the presence of the voltage offset in the differential amplifier reduces the accuracy of the voltage applied across the memory cell, and consequently the precision with which the logic state of the cell is read. Therefore, there is a need for an improved voltage source circuit that is readily implemented on an integrated circuit, that requires few components, that operates at high speeds, and that provides a highly accurate and stable voltage output and a current output appropriate to support MRAM memory element sensing.
The present invention provides a method and apparatus for measuring the resistance of a resistive memory element. The resistance is measured by applying a controlled voltage across a resistive memory element and ascertaining the level of current that consequently flows through the resistive memory element. According to the present invention, an improved differential amplifier, including a compensation circuit, is used in a feedback configuration to set a standard voltage across a resistive memory element. The compensation circuit reduces the effect of differential amplifier offset and yields a more precise standard voltage across the resistive memory element.
In one aspect, the invention includes a memory element voltage supply including a transistor with a gate adapted to control an electrical current through the transistor in relation to a voltage applied to the gate. The transistor also includes a source (or drain depending on device polarity) operatively connected to the resistive memory device. The voltage supply includes a differential amplifier with a non-inverting input operatively connected to a source of a reference voltage, an inverting input, a first output switchingly connected to the gate of the transistor, and a second output. The supply further includes a compensation amplifier with a third input adapted to receive and store a first compensation voltage, a third output operatively connected to the first output, and a fourth output operatively connected to the second output. Also included is a feedback line including a switch adapted to switchingly connect the second input to the source (or drain) of the transistor.
In operation the voltage supply first enters a calibration phase. During the calibration phase, a first compensation voltage is stored at the third input of the compensation amplifier. Generally, this third input includes a capacitor which is charged to store the first compensation voltage. The compensation voltage stored reflects the magnitude of an offset voltage that exists in the differential amplifier when no differential voltage is applied across its inputs; that is, the compensation voltage reflects an error that exists in the output of the differential amplifier. After the calibration phase, the voltage supply enters a measurement phase. In the measurement phase the error voltage, stored on the capacitor, is used as an input to the compensation amplifier. The resulting output of the compensation amplifier counteracts the erroneous offset voltage that would otherwise be included in the output of the differential amplifier. As a result of this corrective action, the output of the differential amplifier is more accurate, and a more precise measurement may be made of the resistance of the resistive memory element than would otherwise be possible.
These and other aspects and features of the invention will be more clearly understood from the following detailed description which is provided in conjunction with the accompanying drawings.